Coaxial filter



I y 1965 H. c. TURNAGE ETAL 3,185,944

COAXIAL FILTER Filed Oct. 24. 1961 low n m N 0 H. v

INVENTORS Houomzo C.Tu|2 uAGr-; Beans o ssEN ATTORNEYS United StatesPatent M 3,1353% CGAXEAL FELTER Howard 1. Turnage, ienna, Va., and BerndVossen, Beverly Hills, Califi, assignors to Melpar, lnc.., Falls Qhurch,Va, a corporation of Delaware Filed Oct. 24, 1961, er. No. 147,349 4Claims. (6]. 333']3) The p esent invention relates generally to coaxialmicrowave low pass filters and more particularly to a filter employingan outer conductor of varying spacing from the inner conductor.

It is known that low pass coaxial, microwave filters are obtainable byemploying a plurality of cascaded coaxial sections having alternatelyhigh and low characteristic impedance. In the past, these sections havebeen constructed by providing variations in the diameter of the coaxialcable inner conductor while maintaining the outer conductor diameterconstant. This structure has fre quently necessitated utilization of avery thin central con ductor upon which is mounted a plurality ofrelatively massive, metal annular members. Generally, there is nothingto prevent movement of the annuli longitudinally of the inner conductoraxis and .a certain amount of transverse movement is also possible.

When the prior art filter is subjected to shock or vibration along itsaxis, the inner conductor is frequently broken or stretched, causing avariation in the filter characteristics. Such poor performance of theunit results from the ability of the massive annuli to translate alongthe filter longitudinal axis. Attempts to solve the inadequacies of theprior art by driving retaining pins through the annuli or filling thevolume between the annular sections with dielectric material provedunsatisfactory. Such practices either failed to cure the malfunction orresulted in a highly complicated assembly technique.

The prior art filter was not satisfactory for large scale productionbecause of the difiiculty associated with as sembly. The annuli andtheir surrounding di-electric sleeves, must of essence, be closelyfitted into the outer conductor. Attempts to slide these annuli, whenattached at discrete spacing to the small diameter inner conductor, intothe outer conductor, caused stretching of the inner conductor. Thisresulted in distortion of the discrete spacing between annuli assembliesto a degree which precluded repetition of the predicted and desiredelectrical results. After assembly, no means could be taken to corrector repair electrical deficiencies without destroying the innerconductor/annuli assembly, the most costly por tion of the coaxialmicrowave filter.

The prior device was also subject to transverse movement of the massive,centrally mounted annuli if they were not snugly fit against outerdielectric members. Transverse movement of the annular conductors alsoresulted in stretching and breaking of the inner conductor. In addition,it results in significant variation of filter impedance at the time theunit is subject to transverse shocks or vibrations, even if there is nopermanent damage to the unit.

The present invention obviates these disadvantages of the prior art byemploying a coaxial filter having an outer conductor that varies ininterior diameter. The inner conductor is of constant diameter. Aplurality of stacked annular members of varying inner diameter areemployed as the outer conductor to obtain the successive high and lowcharacteristic impedance sections of the filter. The outer conductor ismaintained in place by retaining rings at either end of the stackedannuli to restrain longitudinal movement. Transverse movement isconstrained by a sleeve within which the constant outer diameter annuliare inserted. Thus, the annuli are not supported by the Bidfidi FatentedMay 25, E965 fine center conductor but are maintained in situ by contactof the outer periphery with the sleeve interior. The annuli innerdiameter, at its smallest periphery, is greater than the inner conductordiameter by the distance necessary to ellect the desired characteristicimpedance sections. Because it may be desirable to restrain all possibletransverse movement of the inner conductor, it can be provided with aninsulator which the annuli sections of minimum inner diameter engage.

The stacked annular members comprise a first section of small innerdiameter and a second section, extending from the first, which has aconsiderably greater inner diameter but the same outer diameter as thefirst section. This structure is easily fabricated on a lathe to veryclose tolerances. The entire unit is easily assembled by stacking theannuli within the sleeve around the inner conductor. The annuli may bemanufactured of separate sections, depending on the tolerance build-uppermissible by the degree of accuracy required by the electricalfunction of the completed filter.

Accordingly, it is an object of the present invention to provide a newand improved low pass microwave filter having great mechanical strengthand which is able to withstand a great deal of shock and vibration withno deleterious effects.

It is an additional object of the present invention to provide a new andimproved microwave, coaxial low pass filter that is manufactured togreat degrees of accuracy to insure consistency in filtercharacteristics.

A further object is to provide a new and improved microwave, coaxial lowpass filter that is easily and inexpensively manufactured, inexpensiveto assemble, requires minimum servicing and yet is manufactured'to greataccuracy.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawings,wherein:

FIGURE 1 is a sectional view of the filter according to a preferredembodiment of the present invention;

FIGURE 2 is a sectional view of FIGURE 1 taken along the lines 22; and

FIGURE 3 is a sectional view of FIGURE 1 taken along the lines 3-3.

FIGURE 1, a sectional view of the preferred embodiment of the presentinvention, comprises an outer, rigid sleeve 11 in which a plurality ofannular, electrically conducting members 12 are stacked between endbrackets 21 and 22. Each annulus 12 includes a first segment 23 having arelatively small aperture and a second segment or ring 24 having aconsiderably greater aperture than the first, the second segment thusforming an extension of the first. The annular members 1 are positionedand maintained within sleeve ii to contact each other and therebyestablish an electrical connection of negligible DC. or A.C. impedancebetween end rings 21 and 22 and coaxial connector shells 37 and Theouter diameter of each annulus i2 is substantially equal to the innerdiameter of sleeve 11 so it fits snugly therein to obviate any possiblemovement between the various elements. A further annulus 25, having thesame inner diameter as segments 24 of annular members 12, is insertedbetween the sections 23 of the two rings 12, located most centrallywithin sleeve 11. Annulus 25 is necessary when an even number of coaxialsections is utilized in the low-pass filter to effect the desiredimpedance variation of the successive sections.

Cable 26 extends through the apertures in annuli 12 and is maintained inconstant position with respect thereto by the tension exerted on it byend retaining members 29 and 31. Cable 26 includes an inner electricconductor 27 surrounded by an'insulating sleeve 28, preferablyconstructed of Teflon, to which it is bonded, as seen best in FIGURES 2and 3. The inner diameter of segments 23 is preferably the same as theouter diameter of insulation 28 so a snug fit between them exists.Cornpression of Teflon sleeve 23, to a certain extent, by segments 23 isnot detrimental and is frequently advisable to restrict transversemovement of the inner conductor cable by annuli 12. Complete constraintof transverse movement is provided when the inner and outer surfaces ofannuli 12 are contacting insulation 23 and the interior of sleeve 11,respectively. This is highly desirable in order to maintain the filterimpedance constant regardless of any physical shock to which the unit isubjected. Longitudinal movement of the stacked sections 12 relative tosleeve 11 is prevented by compression exerted thereon by cup shaped endrings 21 and 22 by virtue of threaded sleeves 36. Rings 21 and 22relieve stress from annuli members '34 to prevent cold flow of theTeflon di-electric material used.

Annuli members 3 constructed of electrical insulating material (Teflon)maintain members 31 in place. Conductor 27 of cable as is crimped to theouter portions of members 31 and shoulders 35 of members 31 preventinward movement of members 31. Conductor 27 is stretched between members31 at assembly, Members 31 are retained in place by the inner surfacesof annuli 21 and 22 and sleeves 36. The outer surfaces of sleeves 36 arethreaded to engage matching threaded portions of the inner surface atthe ends of sleeve 11.

Connectors 32 and 33 are identical, conventional coaxial connectors (itis to be understood other types may be used), for electricallyconnecting the filter to its circuitry. Sleeve 33, connector 39 andbushing 41 are usual and common to this type connector. Contacts 39 areattached, by soldering, to the projections of members 36 to complete theelectrical circuit of the inner conductor. Connector shells 37 and 43are screwed in place with their internal threads which match theexternal threads at the ends of sleeve 11. Electrical contact. betweenshells 37 land 43 and sleeves 38, 36, and annuli 21 and 12 provide apath of negligible D.C. resistance. Accordingly, sleevell may beconstructed from any suitable material, not necessarily a conductor.However, if a great deal of shielding is necessary or if there is anypossibility of a slight separation between the components forming outerconductor 12 between the extremities of end assemblies 32 and 33, it ispreferable that sleeve 11 be a conductor.

Disposed within member 37 and concentric therewith is a conventionalcoaxial interior female connecting element, soldered on its interiorsurface to end retainer 31.

Assembly of the filter is accomplished as follows: the insulation 28 ofcable 26 is removed for an adequate length at one end to exposeconductor 27 which is crimped at the protruding section of one of themembers 31. Annular members 34, 21, 12, 25, etc., are slid over the wirein their proper sequence. This assembly is slid into rigid sleeve 11after one of the threaded sleeves 36 has been screwed into oneend ofrigid sleeve 11. The

second threaded sleeve 36 is screwed into the other end i of rigidsleeve 11 retaining this assembly within rigid sleeve 11. The insulation28 is now removed, flush with the inside face of annulus 22 to exposethe remaining length of conductor 27 of cable 28. Member 29 is slid'over this exposed portion so that its shoulder 35 is seated and theseassemblies are screwed to rigid sleeve 11 to complete the filter.

It should thus be apparent that there is provided a low pass microwavecoaxial filter having an outer conductor of alternately large and smallinterior diameter. By utilizing the outer conductor, rather than theinner conductor, to obtain variation in the ratio of inner to outerconductor diameter a considerable increase in mechanical strength isobtained. Also, it is possible to machine annuli 12 to a great degree ofaccuracy because they are separate from inner conductor 26 during theentire fabrication process. The present invention is easily assembledbecause of the relative ease with which slugs 12 are inserted withinsleeve 11 around conductor 26.

While we have described and illustrated one specific embodiment of ourinvention, it will be clear that variations of the details ofconstruction which are specifically illustrated and described may beresorted to without departing from the true spirit and scope of theinvention as defined in the appended claims.

We claim:

1. A coaxial low pass filter comprising an elongated inner conductor ofsubstantially constant diameter, an outer elongated hollow conductorconcentric with, insulated from and substantially surrounding said innerconductor, said conductors having substantially the same length, firstcoaxial connector means connected to both of said conductors at one endof said filter, second coaxial connector means connected to both of saidconductors at the other end of said filter, said outer conductorcomprising a plurality of discrete annuli of substantially the.

same outer diameter positioned in stacked relationship throughout thedistance between both said connector means and held in placelongitudinally of said inner conductor by both said connector means,said annuli, together in the stacked relationship, including a pluralityof sections having high and low characteristic impedances, said sectionsof high and low characteristic impedance being alternately disposedalong the length of said inner conductor, said sections of lowcharacteristic impedance including annuli having their innermost surfaceseparated from said inner conductor by a relatively short distance toprovide relatively large capacity and inductance with said innerconductor, said sections of high characteristic impedance includingannuli having their innermost Surface separated from said innerconductor by a relatively large distance to provide relatively smallcapacity and inductance with said inner conductor, said elements havingno fixed connection with said inner conductor and being slidablerelative thereto when one of said connector means is detached, a sleevehaving an inner diameter substantially equal to the outer diameter ofsaid annuli and having'its inner surface in contact with the outersurfaces of all of said annuli throughout the distance between saidconnector means so that said conductors bear primarily against saidsleeve and said connector means, wherein one of said annuli comprises: acup-like member having a cylindrical base segment with a bore throughwhich said inner conductor extends, a cylindrical flange segmentextending from one face of said base segment; said base segment of saidcup forming one of said high characteristic impedance sections, saidflange segment of said cup forming one of said low characteristicimpedance sections. 7

2. The low pass filter of claim 1 wherein an insulating sleeve iscarried by and surrounds said inner conductor, said insulating sleevehaving a diameter substantially equal to the diameter of said bore.

3. A coaxial low pass filter comprising an elongated innerrconductor ofsubstantially constant diameter, an outer elongated hollow conductorconcentric with, insulated from and substantially surrounding said innerconductor, said conductors having substantially the same length, firstcoaxial connector means connected to both of said conductors at one endof said filter, second coaxial connector means connected to both of saidconductors at the other end of said filter, said outer conductorcomprising a plurality of discrete annuli of substantially the sameouter diameter positioned in stacked relationship throughout thedistance between both said connector means and held in placelongitudinally of said inner conductor by both said connector means,said annuli, together in the stacked relationship, including a pluralityof sections having high and low characteristic impedances, said sectionsof high and low characteristic impedance being alternately disposedalong the length of said inner conductor, said sections of lowcharacteristic impedance including annuli having their innermost surfaceseparated from said inner conductor by a relatively short distance toprovide relatively large capacity and inductance with said innerconductor, said sections of high characteristic impedance includingannuli having their innermost surface separated from said innerconductor by a relatively large distance to provide relatively smallcapacity and inductance with said inner conductor, said elements havingno fixed connection With said inner conductor and being slidablerelative thereto When one of said connector means is detached, a sleevehaving an inner diameter substantially equal to the outer diameter ofsaid annuli and having its inner surface in contact with the outersurfaces of all of said annuli throughout the distance between saidconnector means so that said conductors bear primarily against saidsleeve and said connector means, wherein: one of said annuli forms oneof said low impedance sections comprising a disc With a bore throughwhich said inner conductor extends; another of said annuli forms one ofsaid high impedance sections comprising a ring with an aperture ofcircular cross section through which said inner conductor extends; anedge of said ring being juxtapositioned with a face of said disc.

4. The low pass filter of claim 3 wherein an insulating sleeve iscarried by and surrounds said inner conductor, said insulating sleevehaving a diameter substantially equal to the diameter of said bore.

References Cited by the Examiner UNITED STATES PATENTS 2,392,664 1/46Gurewitsch 333-73 2,411,299 11/46 Sloan 333-73 2,438,367 3/48 Keister333-73 2,438,913 4/48 Hansen 333-73 2,521,843 9/50 Foster 333-732,543,721 2/51 Collard 333-73 2,641,646 6/53 Thomas 333-73 2,700,1361/55 Devot 333-73 2,877,433 3/59 Devot 333-73 2,944,233 7/60 Pong 333-73HERMAN KARL SAALBACH, Primary Examiner.

1. A COAXIAL LOW PASS FILTER COMPRISING AN ELONGATED INNER CONDUCTOR OFSUBSTANTIALLY CONSTANT DIAMETER, AN OUTER ELONGATED HOLLOW CONDUCTORCONCENTRIC WITH, INSULATED FROM AND SUBSTANTIALLY SURROUNDING SAID INNERCONDUCTOR, SAID CONDUCTORS HAVING SUBSTANTIALLY THE SAME LENGTH, FIRSTCOAXIAL CONNECTOR MEANS CONNECTED TO BOTH OF SAID CONDUCTORS AT ONE ENDOF SAID FILTER, SECOND COAXIAL CONNECTOR MEANS CONNECTED TO BOTH OF SAIDCONDUCTORS AT THE OTHER END OF SAID FILTER, SAID OUTER CONDUCTORCOMPRISING A PLURALITY OF DISCRETE ANNULI OF SUBSTANTIALLY THE SAMEOUTER DIAMETER POSITIONED IN STACKED RELATIONSHIP THROUGHOUT THEDISTANCE BETWEEN BOTH SAID CONNECTOR MEANS AND HELD IN PLACELONGITUDINALLY OF SAID INNER CONDUCTOR BY BOTH SAID CONNECTOR MEANS,SAID ANNULI, TOGETHER IN THE STACKED RELATIONSHIP, INCLUDING A PLURALITYOF SECTIONS HAVING HIGH AND LOW CHARACTERISTIC IMPEDANCES, SAID SECTIONSOF HIGH AND LOW CHARACTERISTIC IMPEDANCE BEING ALTERNATELY DISPOSEDALONG THE LENGTH OF SAID INNER CONDUCTOR, SAID SECTIONS OF LOWCHARACTERISTIC IMPEDANCE INCLUDING ANNULI HAVING THEIR INNERMOST SURFACESEPARATED FROM SAID INNER CONDUCTOR BY A RELATIVELY SHORT DISTANCE TOPROVIDE RELATIVELY LARGE CAPACITY AND INDUCTANCE WITH SAID INNERCONDUCTOR, SAID SECTIONS OF HIGH CHARACTERISTIC IMPEDANCE INCLUDINGANNULI HAVING THEIR INNERMOST SURFACE SEPARATED FROM SAID INNERCONDUCTOR BY A RELATIVELY LARGE DISTANCE TO PROVIDE RELATIVELY SMALLCAPACITY AND INDUCTANCE WITH SAID INNER CONDUCTOR, SAID ELEMENTS HAVINGNO FIXED CONNECTION WITH SAID INNER CONDUCTOR AND BEING SLIDABLERELATIVE THERETO WHEN ONE OF SAID CONNECTOR MEANS IS DETACHED, A SLEEVEHAVING AN INNER DIAMETER SUBSTANTIALLY EQUAL TO THE OUTER DIAMETER OFSAID ANNULI AND HAVING ITS INNER SURFACE IN CONTACT WITH THE OUTERSURFACES OF ALL OF SAID ANNULI THROUGHOUT THE DISTANCE BETWEEN SAIDCONNECTOR MEANS SO THAT SAID CONDUCTORS BEAR PRIMARILY AGAINST SAIDSLEEVE AND SAID CONNECTOR MEANS, WHEREIN ONE OF SAID ANNULI COMPRISES: ACUP-LIKE MEMBER HAVING A CYLINDRICAL BASE SEGMENT WITH A BORE THROUGHWHICH SAID INNER CONDUCTOR EXTENDS, A CYLINDRICAL FLANGE SEGMENTEXTENDING FROM ONE FACE OF SAID BASE SEGMENT; SAID BASE SEGMENT OF SAIDCUP FORMING ONE OF SAID HIGH CHARACTERISTIC IMPEDANCE SECTIONS, SAIDFLANGE SEGMENT OF SAID CUP FORMING ONE OF SAID LOW CHARACTERISTICIMPEDANCE SECTIONS.